1. Field of the Invention
This invention relates to a throttle control system for vehicular internal combustion engine, more particularly to such a system wherein an actuator such as a stepper motor is connected with the throttle valve and the position of the throttle valve is controlled by the actuator in response to the engine operating condition.
2. Description of the Prior Art
In vehicular internal combustion engines it is well known to employ idle speed control techniques for maintaining a constant engine speed during idling. Cruise control technology for maintaining a constant speed during driving is also widely employed. While these two types of control were originally conducted separately using independent systems, technology for combining the control functions in a single system has been proposed as, for example, in Japanese Patent Publication 62(1987)-115749.
In the proposed system using a single apparatus for controlling the throttle valve opening during both idling and cruise driving as well as, in some cases, in response to other engine operation conditions, it has not been completely clarified how reconciliation should be obtained between the two or more throttle positions determined in cases where two or more pertinent engine conditions occur simultaneously.
It is therefore a first object of the invention to overcome the aforesaid shortcomings of the conventional control system by providing a throttle control system wherein the throttle position can be optimally controlled to satisfy both the idle speed control and the cruise control even under engine operating conditions in which the idle speed control region and the cruise control region overlap.
Where the stepper motor is used for opening and closing the throttle valve, however, the characteristics of this type of motor will cause stepwise changes in the throttle position which will in turn cause the amount of intake air to change stepwise, making it impossible to vary the amount of intake air smoothly as can be done when the position of the throttle valve is determined directly by the accelerator pedal. The stepwise changes in the amount of intake air are determined by the bore of the intake air passage (the area of the throttle valve) and the step angle of the stepper motor. The bore of the intake air passage is determined during engine design on the basis of various factors including the desired engine output and, therefore, cannot readily be changed solely for the purpose of eliminating stepwise change in intake air supply. While countermeasures can be taken on the side of the stepper motor such as by increasing the number of motor poles or by increasing the gear speed reduction ratio, there is a limit to the effect that can be obtained by these means.
Therefore, the second object of this invention is to overcome the aforesaid shortcomings of the prior art by providing a throttle control system wherein the position of the throttle valve can be finely controlled when driven by the stepper motor.
Furthermore, the vehicle battery is used as the power source for the stepper motor. This means that the battery voltage must be maintained at not less than a prescribed level for proper operation of the stepper motor. Thus the conventional practice has been to constantly detect the battery voltage and to correct the motor command value accordingly, as disclosed for example in Japanese Laid-open Patent Publication 61(1986)-19946. When this arrangement is used, however, it is necessary for monitoring the battery voltage to convert the voltage sensor output into a digital signal and to compare the converted signal with a reference value. The operation and structure thus become complex.
It is therefore a third object of the present invention to provide a throttle control system wherein decrease in the battery voltage can be easily detected and the command value supplied to the stepper motor corrected accordingly so that the stepper motor operates properly even when the battery voltage has dropped.
The fourth object of the invention is to provide a throttle control system wherein highly optimum throttle position control is realized by varying the pulse rate and the chopping duty of the command signal supplied to the stepper motor.
Further object of the invention is to provide a throttle control system wherein the throttle position is controlled using the stepper motor in such manner that ramping drive of the pulse motor is optimally controlled satisfying real-time operation requirements.
For realizing the aforesaid objects, the invention provides a system for controlling throttle position in a vehicular internal combustion engine including first means for detecting operating condition of the engine including the degree of depression of an accelerator pedal provided on the vehicle floor, second means for determining a target position of a throttle valve provided in an engine air intake passage in accordance with the detected degree of depression of the accelerator pedal, control means for determining a command value in accordance with the determined target position and actuator means connected to the throttle valve for moving the valve in response to the command value. The system is arranged such that the second means further determines an idle throttle position when the engine operation enters an idle region and a cruise throttle position when the engine operation enters a cruise region and the control means calculates the command value by selecting the largest value among the throttle positions determined by the second means.